1. Field of the Invention
The present invention relates to an image forming apparatus having a laser-raster optical writing unit.
2. Description of the Background Art
Typically, image forming apparatuses include an optical scanning unit employing a polygon scanner (or polygon mirror) as an optical deflector. With increased demand for high-speed, high-quality image printing using color image forming apparatuses, the polygon scanner is required to rotate at high speeds with great precision, up to 25,000 revolutions per minute (rpm) or more. Further, the polygon scanner is being subjected to use under increasingly heavier load environments to achieve the desired higher image quality printing, involving use of laser beams having smaller beam spot diameters which in turn requires a relatively greater inscribed circle radii and main scanning direction lengths of polygon mirrors.
Such heavier load increases power consumption of the polygon scanner and adversely affects optical elements such as scanning lens due to heat generated by the high speed of rotation of the polygon scanner, in particular that scanning lens which is disposed closest to the polygon scanner.
Moreover, the heat generated by the polygon scanner may be transferred to an optical housing and then transferred to the scanning lens, or may be radiated to the scanning lens directly to increase temperature. In actual apparatuses, such scanning lens may not be uniformly heated, and thereby its temperature does not increase uniformly over the entire area of the scanning lens. For example, distance from a heat generating source (e.g., the polygon scanner) to the scanning lens, differences in coefficient of thermal expansion of material of the scanning lens, flow patterns of air flow in the optical housing, etc., all affect heat transfer to the scanning lens. Specifically, a clear heat gradient over the scanning lens may appear in a main scanning direction, which is also the long direction of the scanning lens.
This heat gradient along the scanning lens in the main scanning direction may change the shape and refractive index of the scanning lens, by which a beam spot position of the laser beam may fluctuate, and thereby image quality deteriorates. Such problem becomes particularly acute when plastic material having a greater coefficient of thermal expansion is used for the scanning lens.
In addition to the above-described problem, because laser beams for each image color (e.g., yellow, magenta, cyan, and black) are scanned in color image forming apparatuses, temperature variation among optical scanning units used for each color also becomes a problem. Such temperature variation causes deviation in the relative positions of the beam spots of each color, resulting in image-color misalignment.
Further still, the heat generated by a polygon mirror operating under heavy load causes a temperature increase, and such temperature increase may induce micro-movement of components of rotating members (especially, polygon mirror having greater mass ratio in the rotating member), unbalancing the rotating members and causing them to vibrate. Components of rotating member may include a polygon mirror, a flange to fix a rotor magnet, a shaft, or the like, and such components may have coefficients of thermal expansion that are different or the same. Even if the coefficients of thermal expansion of components are the same, micro-movement of the components of the rotating members and the resultant imbalance may occur at high-speed rotation at high temperature if parts tolerance and mounting/assembly is not precise, further increasing. Such vibration may be further transmitted to and amplified by optical elements (e.g., reflection mirror) in the optical scanning unit, by which banding may occur, resulting in image quality deterioration and noise generation.
In view of such problems, an oscillating mirror using resonance phenomenon has been researched, in an effort to use the oscillating mirror as an equivalent of and replacement for a polygon mirror deflector. An oscillating mirror deflector, having a resonance structure oscillate-able by using sine wave, can reduce its size, and deflection scanning can be conducted using a single oscillating mirror. Such oscillating mirror may be provided with a correction unit to correct magnification ratio of image data in a sub-scanning direction depending on oscillation frequency of oscillating mirror, and a speed changer to change a moving speed of a photoconductor drum in a sub-scanning direction, as disclosed in JP-2006-243034-A. An image forming apparatus employing such oscillating mirror can reduce power consumption, and prevent or suppress temperature increase of the scanning lens disposed in an optical scanning unit, and reduce temperature variation and vibration of the optical scanning units in color image forming apparatuses.
In an image forming apparatus using an oscillating mirror employing resonance phenomenon as a deflector, when a rotation moving speed of photoconductor is changed, or image resolution is changed, a drive frequency Fd of the deflector (i.e., the oscillating mirror) is changed and set to a drive frequency corresponding to an operating condition. However, such drive frequency Fd may not match a resonance frequency Fr of the deflector (i.e., oscillating mirror), by which amplitude of oscillating mirror may become smaller. JP-2005-305771-A discusses a method of solving such problem, in which, by shifting resonance characteristic of the deflector when the drive frequency Fd is changed, the resonance frequency Fr of the deflector which shows a maximum amplitude can be substantially matched to the drive frequency Fd of the deflector. An image forming apparatus employing such oscillating mirror can reduce power consumption, suppress temperature increase of the scanning lens disposed in an optical scanning unit, and reduce temperature variation and vibration of optical the scanning units in color image forming apparatuses.
However, because the oscillating mirror uses resonance phenomenon, a size/shape error of a reflector and bar beam set around the oscillating mirror may change the resonance frequency of the oscillating mirror. Further, a size tolerance for a reflector having a size of several millimeters may need to be on the order of micrometers (μm) or less, but current processing techniques may not be able to satisfy such processing precision requirements. Accordingly, mass-produced oscillating mirrors may exhibit resonance frequency variation, by which image magnification ratio error may occur when an image forming operation is conducted.
Further, environment temperature change may change rigidity such as Young's modulus of bar beam. As a result, the resonance frequency may vary, and thereby magnification ratio error may fluctuate due to the change of rigidity of the bar beam over time.
Even in JP-2006-243034-A, resonance frequency variation of oscillating mirrors may become a problem, wherein such resonance frequency variation pattern may be included when the oscillating mirrors are manufactured and caused by ambient temperature changes over time. Such resonance frequency variation may cause fluctuations in image magnification ratio during image printing operations, depending on installation environment of the image forming apparatus and when a greater number of image are printed, for example.
Further, in JP-2005-305771-A, resonance frequency of the oscillating mirror may be changed to match the resonance frequency and a drive frequency of the oscillating mirror with each other. Such frequency matching may be conducted using an electrical resistance element, in which the electrical resistance element is used to change the temperature of a twisted spring to change the resonance frequency of the oscillating mirror. However, such method may take a longer time, and image forming operations cannot be conducted during such frequency matching process.
Further, even if the drive frequency and resonance frequency can be matched with each other by changing the image resolution, the resonance frequency of oscillating mirror of resonance type may change due to ambient temperature changes, by which a desired amplitude may not be obtained due to temperature changes in the image forming apparatus over time.